Patterns in the abundance and biomass of fauna inhabiting rhodolith beds of varying structure: the potential role of facilitation cascades

Abstract Rhodolith beds are globally distributed benthic habitats that play a key role in supporting associated faunal communities and serve as important biodiversity hotspots. Their ecological importance has been attributed to their ability to increase habitat complexity. In this context, rhodoliths are considered habitat-forming species (primary facilitator) that potentially support the establishment of further habitat providers, such as epiphytic macroalgae (secondary facilitator), which contribute to structural heterogeneity, and possibly, to facilitation processes. This study explored how the structural characteristics of rhodoliths and associated epiphytes affect the abundances and biomasses of epifaunal assemblages across five rhodolith beds in the Canary Islands. We identified 11,222 epifaunal organisms, of which 53.5% belonged to Mollusca, 30.6% to Amphipoda, and 6.1% to Decapoda, as the most abundant taxonomic groups. Epiphytic loads had a significant influence, positively affecting the abundance and biomass of decapods, brachyurans, amphipods, and polychaetes. Molluscs were otherwise only affected by rhodolith structural attributes (size of nodules). These findings suggest differentiated responses among faunal groups to distinct components of habitat structure, and may serve to generate hypotheses about multi-level facilitative interactions to be tested in experimental studies. If different structural elements influence specific faunal assemblages, conservation strategies should not only preserve rhodoliths (primary facilitators), but also maintain the positive ecological interactions they support via secondary facilitators.